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Piwoslaw 06-24-2009 03:08 AM

Tire size vs. hub width and weight limits
 
I still have some time before getting new tires (both sets are relatively new, and with my driving that won't change much), but I'm starting to research the option so that I'm ready when it's time.
Peugeot suggests two tire sizes for its 307 model: 195/65R15 and 205/65R15. I bought the car with 195/65R15 summer tires, for winter I bought 185/65R15. My hub size is 6x15". I read somewhere about suggested hub width for certain tire sizes and it said that for 185/65R15 tires the suggested hub is 5.5", but 4.5"-6.0" is still acceptable. I looked around for 5.5x15" hubs, but no luck. Only PSA (Peugeot and Citroen) uses the 4x108mm bolt pattern, which makes finding less-standard sizes almost impossible. Does seating a 185/65R15 tire on a 6" hub and on a 5.5" hub make any difference?

When the time finally comes to replace the summer tires, I would like a low RR set, maybe even narrower than 185/65R15. This is where the real problems start. First, a 175mm-wide tire probably shouldn't sit on a 6" hub.
Second, as the tire gets narrower, it should get taller, but the only 175/80R15's I can find here are for 4x4's and delivery vans, no LRR's :( To get a larger diameter I could get 16" hubs and tires, but the trend is usually wide tires on large rims, so getting something narrow may be hard.
Third, narrower tires usually have lower max psi (44 instead of my 51).
Fourth, narrower tires have a lower weight limit. I must be cautious with this, since my car weights 1.5 tonnes, and has a max of 2 tonnes. That's at least 500kg on each corner, but the weight isn't always evenly distributed. My winter 185/65R15 tires have a weight limit of 560kg, so I probably come close to that in the rear when carrying cargo.

So any suggestions? Should I just look for LRR 185/65R15's, or something else?

Thanks,
Adam

CapriRacer 06-24-2009 08:47 AM

Adam,

First, in passenger car tires, tire size determines load carrying capacity. Peugeot's haven't been imported into the US for quite a while, so my books only have older entries for Peugeot's - and a 307 model isn't one of them.

Be aware that even within a given vehicle model, there may be different tire sizes used - and sometimes this is a reflection of the different vehicle weights.

So the first step is to find your tire placard that lists the original tire size and the proper pressure for that size. It's a law in the US to have this placard, but my understanding is that tire placards are pretty universal worldwide.

So using a 185/65R15 is actually going down in load carrying capacity and that increases the risk of a load related tire failure, which sometimes has tragic results. I don't recommend you continue this practice.

Second, a tire's rolling resistance is only slightly connected to tire size. It is more connected to the amount of deflection (load vs inflation pressure), the amount of material being deflected (basically the amount of tread rubber), and the material properties of the material being deflected (mostly the hysteresis of the tread rubber)

Within a given tire line, rollling resistance coefficient doesn't vary much between tire sizes, assuming the load on the tire is the same. You should take this to mean that there isn't any real advantage to going down in tire size. In fact tires with larger load carrying capacity tend to have better rolling resistance coefficients - and while width is a factor, it seems to be small compared to overall load carrying capacity. There is a school of thought that says that for good fuel economy, you want to get the tire with the largest load carrying capacity you can.

But differences between tire lines can be HUGE!! But there is a price to be paid.

The classic technology triangle is Rolling Resistance / Treadwear / Traction (especially wet traction)

BTW, most car folks use the term "hub" to mean the part that the wheel attaches to, and not the "rim" or "wheel" which is what attaches to the "hub" and is the thing the tire is mounted on. Tire specs routinely refer to the dimension in question as "Rim Width".

Don't pay much attention to the max pressure listed on the sidewall of the tire (at least for Standard Load passenger car tires). There isn't a universal rule about how this is derived. I talk about it here:

Barry's Tire Tech

One of the problems you are going to face is availablility of information about the specific tires you can find in your locale.

First, OE tires (tires delivered to vehicle assembly plants) tend to have better RR than tires designed strictly for the replacement market. OE tires ARE available in the replacement market, but tend not to be identified as such. Further, since a given vehicle will have a limited range of tire sizes that are applied - and the tire specs will be different between vehicle models - even tires within a line of OE tires will vary all over the place for RR.

Even further, different parts of the world may have the same tire name, but the tire itself will be geared towards a different performance goal. Put another way, you can't be sure that a "Goodstone FireEagle GT" is EXACTLY the same tire in the US as it is in, say, Australia.

Not to mention that certain tire lines won't be available on a worldwide basis. There are country specific regulations that might prevent this.

OK, that addresses the basics of your posting - and I'm sure others will be able to point out tires that might suit your purposes.

Blue Bomber Man 06-24-2009 06:38 PM

Capri:

On the research that I have read the best range for low RR tires is around 15-16 inches in general. However the best documented tire I could find was the B381 Bridgestone, which was a 14 inch tire

CapriRacer 06-25-2009 06:50 AM

Quote:

Originally Posted by Blue Bomber Man (Post 111986)
Capri:

On the research that I have read the best range for low RR tires is around 15-16 inches in general. However the best documented tire I could find was the B381 Bridgestone, which was a 14 inch tire

On a practical basis, most low RR will appear in the smaller tire sizes. That's because vehicle manufacturers are targeting the "fuel economy" segment with small cars and they specify tires with lower RR. Obviously larger cars are heavier and less fuel economy comes along for the ride - and this has a major affect on overall fuel economy - much more than tires as a vehicle component has an effect.

However, if you look at the sizing aspect by itself, larger load carrying capacity has a minor effect compared to the tread compound itself, but larger tires are directionally better.


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